It is common to analyze trace elements by injecting samples containing the trace elements into a plasma, and then sampling the plasma into a mass analyzer such as a mass spectrometer. Usually, but not necessarily, the plasma is created by a high frequency induction coil encircling a quartz tube which contains the plasma; hence, the process is usually called inductively coupled plasma mass spectrometry or ICP-MS. An example of ICP-MS apparatus is shown in U.S. Pat. Nos. 33,386 reissued Oct. 16, 1990 and 4,746,794 issued May 24, 1988, both assigned to the assignee of the present application.
Although ICP-MS systems are widely used, they have for many years suffered and continue to suffer from the serious problems of non-uniform matrix effects, and mass bias. Matrix effects occur when the desired analyte signal is suppressed by the presence of a concomitant element at high concentration. The problem occurs when a large number of ions travel through a small skimmer orifice into the first vacuum chamber containing ion optics. The ions create a space charge existing primarily in the region between the skimmer tip and the ion optics and also in the ion optics. The space charge reduces the number of ions which travel through the ion optics. A sample to be analyzed will usually contain a number of other elements in addition to the analyte element (i.e. the analyte element is embedded in a matrix of other elements), and if such other elements (often called matrix elements) are present in high concentration, they can create an increased space charge in the region between the skimmer tip and the ion optics. This reduces the transmission of the analyte ions.
In addition, in a conventional sampling interface, the ions travel through the interface at the speed of the bulk gas flow through the interface, and since all the ions have substantially the same speed, their energy increases with their mass (to a first approximation). If a matrix or dominant element is present in large concentration and has a high mass, it will persist through the space charge region more efficiently than other elements because it has a higher ion energy, and will therefore become the major space charge creating species. This worsens the space charge effect and reduces the transmission of low mass (low energy) ions more than that of high mass (high energy) ions. This effect is described in a paper entitled "Non-Spectroscopic Inter Element Interferences in Inductively Coupled Plasma Mass Spectrometry (ICP-MS)", by G. R. Gillson, D. J. Douglas, J. E. Fulford, K. W. Halligan, and S. D. Tanner, Analytical Chemistry, volume 60, 1472 (1988), and in a paper entitled "Space Charge in ICP-MS: Calculation and Implications" by S. D. Tanner, Spectrochimica Acta, volume 47B, 809 (1992). Therefore the matrix suppression effect tends to be non-uniform, i.e. it varies with the mass of the dominant element and with the mass of the analyte element. The non-uniformity is undesirable since sensitivity is reduced for some masses, and since corrections for changes in sensitivity are mass dependent (i.e. different for each element). Further, since ion transmission is dependent on mass, there will be small but significant changes in measured isotope ratios, particularly for light isotopes.
Even without a dominant matrix element, the space charge tends to create a non-uniform mass response, in that high mass analytes are transmitted through the skimmer to the ion optics and through the ion optics more efficiently (because of their higher kinetic energy) than low mass analytes. This is called mass bias, and it is also undesirable, for the same reasons.
One way of dealing with the space charge problem, as disclosed by P. J. Turner in an article entitled "Some Observations on Mass Bias Effects in ICP-MS Systems", disclosed in "Application of Plasma Source Mass Spectrometry", editors G. Holland and A. N. Eaton, published by the Royal Society of Chemistry, United Kingdom, 1991, is to apply a high voltage to accelerate the ion beam emerging from the skimmer orifice, as close to the skimmer orifice as possible. Since space charge varies inversely with the velocity of the ions, if the ions can be accelerated, the resultant space charge will be reduced. The Turner system works well in reducing space charge effects. However it suffers from the disadvantages that it may create large energy spreads which can degrade the mass spectrometer resolution; the high voltage creates a greater likelihood of electrical discharges which can cause excessive continuum background noise; and (as do conventional ICP-MS systems) it requires large and expensive vacuum pumps.
It is therefore an object of the present invention to provide an improved method and apparatus for plasma mass analysis, in which matrix effects are made more uniform and mass bias is reduced, effectively by reducing space charge effects.